1. Field of the Invention
The present invention relates to an electrochemical reactor unit composed of an electrochemical reactor cell stack, and to an electrochemical reaction system, such as a solid oxide fuel cell, made up of these electrochemical reactor cell stacks, and more particularly relates to an electrochemical reactor unit, electrochemical reactor module, and electrochemical reaction system that make use of tube-type electrochemical reactor cells and that afford a marked increase in the output per unit of volume.
The present invention provides a novel technique and a novel product relating to an electrochemical reactor unit and reactor module that can be used to advantage as an electrochemical reaction system such as an environmental purification apparatus or a clean energy source, and to an electrochemical reaction system in which such a reactor module is utilized.
2. Description of the Related Art
A solid oxide fuel cell (hereinafter referred to as SOFC) is a typical example of an electrochemical reactor. An SOFC is a fuel cell in which a solid oxide having ion conductivity is used as an electrolyte, and a solid oxide having electron conductivity is used as an electrode. The basic structure of an SOFC usually comprises three layers: an air electrode (cathode), electrolyte, and a fuel electrode (anode), and is usually used at a temperature between 800 and 1000° C.
When a fuel gas (such as hydrogen, carbon monoxide, or a hydrocarbon) is supplied to the anode of the SOFC, and air, oxygen, or the like is supplied to the cathode, a differential is produced between the oxygen partial pressure on the cathode side and the oxygen partial pressure on the anode side, and this produces voltage between the electrodes according to Nernst's formula. Oxygen becomes ions at the cathode, which move through the solid electrolyte to the anode side, and the oxygen ions that reach the anode react with the fuel gas and release electrons. Accordingly, if a load is connected to the anode and cathode, electricity can be taken off directly.
For SOFCs to see practical application in the future, their operation temperature needs to be lowered (600° C. or lower), and an effective means to this end is to use of an electrolyte material having high ion conductivity and to use an electrolyte thin film. A thin film of electrolyte can be obtained by using a support made of an electrode material, so a great deal of research has gone into this, particularly for anode support-type cells.
If the operating temperature could be lowered to between 500 and 600° C., we would be able to use less expensive materials and bring down the operating costs, and this should make SOFCs more versatile. Novel anode and cathode materials have been proposed in the past, and it has beer reported that a flat type of SOFC having a high power output of 0.8 to 1 W/cm2 even at a low temperature (600° C.) can be constructed (see Z. Shao and S. M. Haile, Nature, 431, 170-173 (2004); and T. Hibino, A. Hashimoto, K. Asano, M. Yano, M. Suzuki, and M. Sano, Electrochem. Solid-State Lett., 5 (11), A242-A244 (2002)).
However, the anode support-type SOFCs having a high power output that have been reported on up to now are flat, and under conditions of a harsh operating cycle, a problem is that the cell can sometimes fail. The reason for this is that the nickel-cermet that is commonly used undergoes a large change in volume due to temperature changes or redox atmosphere cycling, and this causes strain in the cell and leads to failure.
Consequently, if SOFCs are to see practical use, one of the greatest technological challenges is to increase the size and build stacks while still preserving the performance of the flat cell. It is also important, in terms of boosting performance, to reduce thickness and control the electrode structure of the anode support substrate, but up to now it was difficult to reduce the thickness and raise porosity with a flat cell. An SOFC structure composed of a tubular cell has also been the subject of research as a structure that could replace flat cells (see Japanese Patent Application Laid-Open No. 2004-335277).
With stacks or bundles composed of tube-type cells that have been proposed in the past, the structure was such that, for example, tube-type cells are stably supported by an integrating structure composed of a cathode material, and an electrode integrating sheet or the like is used to collect current from the anode and cathode portions.
However, with the structure of existing tube-type cell bundles and stacks, as the integration density of the tubes rose, it became more difficult to control the temperature because of heat generated during operation of the stack, and the air pressure loss increased. Therefore, there has been an urgent need in this field of technology for the development of a new way to solve these problems.
In the midst of this situation, the inventors conducted diligent research in light of the above-mentioned prior art and aimed at developing an SOFC structure with which the problems encountered with the above-mentioned conventional product members can be effectively solved, and new modes for using this structure. As a result, the inventors perfected the present invention upon conducting further research following new findings such as a handle structure having a heat dissipation function and current collecting function in which tube-type cells with a small diameter are arranged, the fact that a method for stacking these cells can be constructed, and the fact that an electrochemical reactor module can be constructed as an electrochemical reaction system that utilizes these stacked units and affords a reduction in the operating temperature.